1. Field of the Invention
The present invention relates generally to high performance CMOS device structures with a mid-gap metal gate, and more particularly pertains to high performance CMOS device structures with a mid-gap work function metal gate wherein an epitaxial layer is used for a threshold voltage Vt adjust/decrease for the PFET area, for large Vt reductions (˜500 mV), as are required by CMOS devices with a mid-gap metal gate.
2. Discussion of the Prior Art
The problems and issues associated with the design of CMOS devices with a mid-gap work function metal gate are well known and recognized in the prior art (e.g. E. Josse and T. Skotnicki, IEDM 1999).
In the fabrication of high performance CMOS devices with ultra-thin gate dielectrics and polysilicon gates, the depletion layer formed in the polysilicon gate in inversion bias becomes a significant fraction of the gate capacitance and degrades the device performance.
The use of a metal gate in these CMOS devices alleviates this problem. Two different metals with appropriate workfunctions can be used, a first metal with a first workfuction for the PFET area and a second metal with a second workfunction for the NFET area. However, this approach adds significant cost and complexity to the process. Alternatively, the same metal can be used for the gate of both the PFET area and NFET area with a mid-gap workfunction. For CMOS mid-gap workfunction metal gates, the threshold voltage Vt for both the PFET area and the NFET area become unacceptably high. The threshold voltage has to be adjusted downwardly by adding p-type dopant to the surface of the PFET area and n-type dopant to the surface of the NEFT area.